The invention pertains to long range electronic article surveillance and tracking and, more particularly, to a high sensitivity, lightweight homodyne transceiver for use in an RFID or similar reader for accurately reading data from and/or writing data into tags attached to a multiplicity of items rapidly passing a checkpoint.
Many commercial applications require accurate identification of packaged items in transit or inventory. This is often accomplished by placing machine-readable identification tags on the packages such as barcodes or magnetic stripes. But these methods cannot identify packages, which are not visible to the reader. It is also sometimes necessary to change an item""s identification characteristic such as its shipping destination or cost. Such identification changes require that a reader write data into the package""s tags or labels. Radio frequency identification (RFID) systems, using readers and tags, are currently available for performing these tasks on a variety of items, which are hidden from view in bags, boxes or totes.
The reader in these RFID systems is a transceiver whose transmitter activates an RFID tag. RFID tags are electronic devices that incorporate specific and typically unique identification numbers. These embedded numbers may be xe2x80x9creadxe2x80x9d by an interrogating radio frequency (RF) transceiver (i.e., transmitter/receiver) system. The RFID tags are generally attached to objects to be identified and/or tracked. These tags are transponders which may be either active (powered by an on-board power source such as a battery) or passive (acquiring energy for operation from the incident RF signal.) Passive tags generally have fewer components than do active tags, making them smaller and less expensive.
The identification number is generally contained in a non-volatile memory device within the RFID tag. When properly activated by an RF field, a passive RFID tag modulates its impedance, causing back-scattering of the RF energy field in its vicinity. The receiver portion of the reader then detects the tag""s identification number from within this back-scattered field, thereby identifying all pertinent characteristics of the item to which the tag is attached.
Active RFID tags, on the other hand, have far greater flexibility of design, ranging from a simple battery which powers the ID-containing memory device to complete, active transponder systems using transmitters.
Because of cost considerations, the vast majority of system readers use passive tags and, therefore, receive tag data in the back-scatter mode. Consequently, the reader receiver functions as a homodyne or zero base-band frequency detector. Homodyne receivers utilize a zero beat principle, in which the local oscillator""s frequency is identical to that of the carrier. Attempting to detect the back-scattered signal""s amplitude modulation envelope with a single detected channel will fail in the homodyne environment. This is because the product detector nulls when the tag is at intervals where the back-scatter receiver""s carrier phase at the detector is different from that of the transmitter by odd phase multiples of 90 degrees. Circumventing this problem requires that there be at least one other detector where the phase relationships between carrier and local oscillator differ from the first by 90 degrees. This dual product detection or demodulation scheme is generally referred as xe2x80x9cI/Q Demodulationxe2x80x9d.
Presently, long range systems utilizing passive tags operate in and above the UHF frequency range. Present generation receiver architecture follows one of two basic approaches. In the first approach, a conventional I/Q receiver is used. An I/Q receiver provides two demodulated outputs which are:
The xe2x80x9cIxe2x80x9d output which is a result of product detecting the received signal against an in-phase local oscillator signal, while the xe2x80x9cQxe2x80x9d output is a result of product detecting the received signal against a local oscillator signal with a phase shift of 90 degrees.
Conventional I/Q receivers of the prior art typically utilize couplers, circulators, power dividers and high level mixers. This approach is costly, bulky and has numerous problems, the most serious problem being related to severe local oscillator isolation. If the local oscillator leakage level approaches the input compression level of a conventional mixer, the received backscatter signal will be. xe2x80x9ccapturedxe2x80x9d or xe2x80x9cWiped outxe2x80x9d, thereby rendering the receiver useless. Since poor antenna matching invariably causes severe local oscillator power reflection, it is absolutely imperative that the antenna be perfectly matched for this type of system to function properly.
The second architecture approach for passive tags utilizes tapped transmission lines with a minimum of four detected channels. While these receivers generally perform better than the conventional I/Q receivers, their large distributed transmission line and extra receiver channels add excessive bulk and cost to the overall system. With this design small, handheld readers are almost impossible to fabricate.
U.S. Pat. No. 5,784,686 for IQ COMBINER TECHNOLOGY IN MODULATED BACKSCATTER SYSTEM, issued to You-Sun Wu, et al. describes a homodyne receiver having two outputs: the in-phase or xe2x80x9cIxe2x80x9d output and the out-of-phase or xe2x80x9cQxe2x80x9d output. In the WU, et al. system, the modulated back-scattered signal is composed of an informational signal modulated onto a single-frequency sub-carrier signal. To demodulate the back-scattered signal, the I and Q outputs are combined using an IQ combiner. The IQ combiner introduces a 90 degree phase shift with respect to the frequency of the sub-carrier signal. The outputs are then combined.
U.S. Pat. No. 5,936,527 for METHOD AND APPARATUS FOR LOCATING AND TRACKING DOCUMENTS AND OTHER OBJECTS, issued to Marvin Isaacman, et al., teaches an apparatus and method for a document control system using passive RFID tags attached to documents. ISAACMAN, et al. utilize a plurality of local exciters to interact with the passive RFID tags. The system is under the control of a personal computer.
U.S. Pat. No. 6,046,683 for MODULATED BACKSCATTER LOCATION SYSTEM, issued to Alex Pidwerbetsky, et al. teaches a system utilizing RFID tags whereby items may be located. An interrogator transmits a signal to one or more RFID tags which, in turn, responds by modulating the RF field via conventional back-scattering or by generating a sub-carrier signal that modulates the sub-carrier and forms a reflective signal. The RFID tag""s relative direction and velocity relative to the interrogator is determined using analysis of any Doppler shift.
None of these references teaches or suggests the simple, homodyne transceiver of the instant invention. The inventive receiver, unlike the prior art, utilizes specific characteristics of a xe2x80x9clumped networkxe2x80x9d in combination with an amplitude/phase detector to form an I/Q receiver. Some of the salient advantages of the inventive receiver are:
That, by using the lumped network approach, its size may be significantly reduced relative to receivers of the prior art.
That, by eliminating couplers and circulators used in the prior art, the receiver will remain fully operational regardless of antenna matching.
That, by utilizing the extremely low loss nine pole lumped network, lower power is required from the transmitter and therefore there is higher efficiency and reliability.
That, by utilizing the extremely low loss nine pole lumped network, the receiver sensitivity will be higher than prior arts.
These inventive design improvements over those of the prior art allow miniature hand-held readers to have effective operating distances at least equal to those of larger base-station types. In addition, a unique switch design not shown in the prior art allows a cost-effective and lower loss implementation of a single-pole, four-throw (sp4t) switch from a pair of single-pole, double-throw (spdt) switches.
In accordance with the present invention, there is provided an improved homodyne transceiver for use in RFID and similar applications. The inventive receiver uses a lumped constant network approach to eliminate costly and bulky couplers, circulators and distributed delay lines. A unique single-pole, four-throw (sp4t) antenna switching arrangement is also used. The inventive receiver combines small size with improved sensitivity and efficiency to provide a practical, low-cost, hand-held reader capable of operation over distances of approximately three to five meters. When used with compatible RFID tags, the inventive system may also be used to alter the information stored within the RFID tags.
It is, therefore, an object of the invention to provide an improved, compact homodyne reader with performance capabilities surpassing those of larger, more expensive systems for use in an RFID-type application.
It is another object of the invention to provide an improved, compact homodyne transceiver, which may be hand-held.
It is a still further object of the invention to provide an improved, compact homodyne transceiver, which may interactively alter the contents of an RFID tag.
It is yet another object of the invention to provide an improved, compact homodyne receiver, which utilizes a lumped network to reduce size and improve reader performance and efficiency.
It is an additional object of the invention to provide an improved, compact homodyne receiver which may use I/Q outputs to help accurately distinguish between closel-yspaced articles.